Field of the Invention
The present invention relates to condition responsive control devices or more particularly relates to control devices which are sensitive to extremely small levels of differential fluid pressure.
Control devices which respond to sensed differential fluid pressure changes are of various kinds and types. Such control devices have frequently included electrical switches, fluid flow controlling valves, or other such devices, operated by differential fluid pressure which evidences the existence of some physical condition.
There are a number of environments in which it is desirable to be able to sense extremely small differential fluid pressures and perform a control function. One example of such an environment is in a typical household refrigerator having a small fan for circulating air across air-chilling refrigeration coils. Moisture in the air tends to condense on the chilling coils and forms ice, or frost, which impedes the ability of the coils to chill the air and thus reduces the efficiency of the refrigerator. The efficiency of the refrigeration unit can be maximized by promptly sensing a build-up of ice or frost on the coils and "defrosting" the refrigerator.
The continued formation of ice or frost on the coils gradually restricts the flow rate of air across them and therefore the presence of ice on the coils is evidenced by a change in differential air pressure across the coils. Accordingly, detecting the differential pressure of the air flowing across refrigeration coils is one way of determining when the refrigerator should be defrosted. Control devices have been proposed for performing this function.
In environments of the character referred to the control device must be operated by forces produced by small differential pressures. In order to effectively respond to small differential pressures the control device must either be constructed so that the differential pressure is distributed over a relatively large area to produce a correspondingly large control operating force, or be capable of responding accurately and reliably to extremely small forces.
Generally speaking, control devices used in mass produced equipment, such as refrigerators, cannot be large because of space limitations in the equipment. Hence, control devices which rely on large pressure areas upon which small differential pressures act to exert actuating forces are not practical.
Reasonably small-sized control devices for sensing differential pressures have frequently utilized a flexible diaphragm across which differential pressure is established to impart operating force to the diaphragm. The diaphragms are usually constructed from a rubber or rubber-like plastic material and are secured in place along their outer peripheries. A plate or pad is generally secured to the diaphragm for transmitting actuating forces from the diaphragm to a control member, such as a valve or switch element. Prior art control devices employing diaphragm-type actuators are disclosed, for example, by U.S. Pat. Nos. 2,229,740 (issued to Helmore on Jan. 28, 1941); 3,066,496 (issued to Jokela on Dec. 4, 1962); 3,300,703 (issued to Gold, et al., on Jan. 24, 1967); and 3,359,388 (issued to Houser, et al., on Dec. 19, 1967).
Control devices employing inflatable actuators have also been proposed. These devices, in essence, have employed balloon-like actuator elements instead of diaphragms and the actuator elements have been resiliently expanded, by applied differential pressure, into engagement with movable control members. The control members in turn have performed control functions by sliding or pivoting movement imparted to them by the actuators. Prior art devices of this general type are disclosed, for example, in U.S. Pat. Nos. 1,942,040 (issued to Wolff on Jan. 22, 1934); 2,795,668 (issued to Pucket on June 11, 1957); and 3,247,341 (issued to Kizilos on Apr. 19, 1966).
Operation of these kinds of control devices by extremely small differential pressures (for example 0.05 inches of water or less which corresponds to 0.002 psi or less) has not been possible because of a number of problems. Firstly, the the mass of the diaphragm, plate and/or the associated components which must be moved during operation of the control device has been so great relative to the available force that such controls have lacked adequate sensitivity. Secondly, the existence of frictional losses in the devices has been relatively great (compared to the available forces). Thirdly, frictional losses have not been consistent from cycle to cycle thus reducing the accuracy and repeatability of the devices. Furthermore, many devices are position sensitive in that the differential pressure levels to which they respond change depending upon the positional orientation of the device (primarily because of the varying effects of gravity on the pressure responsive element and associated components). Still further, many prior art control devices have been constructed so that increasingly greater differential pressures are required to be sensed in order to supply enough force to complete a control function of the device.
The kinds of problems referred to have prevented the introduction of small, highly sensitive, relatively inexpensive control devices for general purpose use in mass produced appliances.